The present invention relates to the production of gas from a coal seam and more particularly to an improved cavitation process wherein a foam, i.e. a viscous and compressible fluid, is injected into a coal seam followed by injection of high pressure gas which is then released to form a cavity in a coal seam.
Many subterranean coal seams have large volumes of hydrocarbon gases, usually including methane, trapped therein. These gases represent a valuable resource if they can be produced economically. Where a coal seam is to be mined later, it is beneficial from a safety standpoint to produce as much of these gases as possible before commencement of mining operations.
Presently, methane and any other gases are produced from the coal reservoirs through wells which are drilled into the coal seam. Once a well is drilled and completed, it is common to treat the coal seam in order to stimulate the production of methane therefrom. Generally, this involves some method of improving permeability of the coal seam. One such commonly used stimulation treatment involves hydraulically fracturing the coal seam generally in the same manner as used with conventional oil and gas bearing formations, see for example, U.S. Pat. No. 4,995,463.
Another technique which has been proposed for stimulating a coal seam is sometimes generally referred to as "cavity induced stimulation". In this technique, a wellbore is drilled through a coal seam and by use of various techniques a cavity is formed within the seam adjacent the wellbore. As the cavity is formed, the vertical stress component which normally acts on the coal above the cavity is partially transferred to the sides of the cavity which, in turn, causes the coal to become loaded inwardly as the cavity is being formed. This increased load would normally be greater than the natural load bearing capability of the coal and the coal will fail and break up into small fragments. As the coal fragments are removed from the cavity through the wellbore, a large cavity is formed, thereby providing a relaxed zone into which existing fractures can open making the coal and surrounding rock more permeable to gas flow. This technique can be repeated until the bearing capacity of the coal equals or exceeds the redistributed stress. The net effect of forming a cavity into which surrounding coal can collect is the production of a highly permeable zone filled with fine grain coal particles. For a more complete description of the mechanics involved in a typical cavity induced stimulation process, see "Cavity Stress Relief Method to Stimulate Demethanation Boreholes" A. K. Alain and G. M. Denes, SPE/DOE/GRI 12843, presented at the 1984 SPE/DOE/GRI Unconventional Gas Recovery Symposium, Pittsburgh, Pa., May 13-15, 1984. The cavity used in the above-described technique can be formed in different ways. For example, in the above-cited paper, the cavity in the coal seam is disclosed as being formed by jetting water from the lower end of a dual drill-type string while using compressed air to remove the resulting coal fragments.
Another known technique which has been used to form a cavity in a cavity induced stimulation method involves the use of compressed air, nitrogen or other available gases. A wellbore is drilled and completed into a coal seam. A tubing string is then lowered into the wellbore and the well annulus is closed. Compressed gas is supplied through the tubing string to build up a high pressure in the coal seam adjacent the wellbore. The wellbore is then opened to suddenly vent the pressure, thereby allowing the gas within the cleats or fractures of the coal seam to expand and produce a back pressure which overcomes the induced hoop stress within the coal. Under proper conditions, the result of the sudden release of gas is that the coal fails and breaks into fragments which are then removed from the tubing string. This process can be repeated until the desired permeable zone is formed within the seam.
While this gas cavitation process has increased the initial methane production in some wells by as much as four to five fold, when compared to wells which were hydraulically fractured, it has also been shown that this stimulation technique has not worked in other wells. As taught in U.S. Pat. No. 5,199,766 this failure may be due to the cleat density being much less than it was in the successfully completed wells and large hoop stresses induced in the coal during the drilling process. The lower cleat density increases the strength of the coal sufficiently that these hoop stresses cannot be overcome with the normal gas cavitation completion techniques. According to that patent, a solvent such as ammonia is injected into the coal seam and allowed to dissolve materials from the cleat structure for a period of time sufficient to weaken that structure. After the cleat structure is thus weakened, the cavity induced stimulation technique has been found to effectively stimulate the coal degasification.
In two other situations the gas cavitation process has also been found to be unsuccessful. The process was not effective in wells which had been previously hydraulically fractured as discussed above. In general, the process has not been considered applicable to previously hydraulically fractured wells, because such wells are cased through the coal seam with the fracturing process, and gas production, occurring through perforations formed in the casing adjacent the coal seam. Similar results occurred where the formation contained highly conductive natural fractures.